Relatively low intensity mercury lamps, long used for outdoor as well as indoor lighting, require a relatively simple ballast inductance and power factor capacitor (PFC) for starting and running the lamp, reigniting it each half cycle of applied alternating current. There are millions of such simple ballast installations in the United States which are inadequate to start and run high intensity discharge (HID) lamps which have greatly improved light intensity and color.
High intensity discharge lamps include, for example, high pressure mercury lamps, high pressure sodium (HPS) lamps, metal halide and other halide lamps. All these HID lamps are difficult or impossible to start and run with the widely installed simple ballast, and many complex starting circuits have been proposed to replace the existing installations. Examples are found in U.S. Pat. Nos. 3,235,769 and 3,235,770, 3,334,270 and 3,383,558. Such prior starting circuits are typically designed for one type of HID lamp and do not work with other lamps.
It is the object of the present invention to provide a decidedly simpler starting circuit with fewer, more reliable components, which can be used in replacement or modification of existing ballast installations, which is effective to start all types of low and high intensity discharge lamps, and which responds to changes in the ignition (cold starting and reignition) voltage requirements of individual lamps, as this voltage varies during and beyond the normal or nominal life of the lamp.